FIELD OF INVENTION

The present invention relates broadly to a method of measuring a frequency response of a slider and a system for measuring a frequency response of a slider.

BACKGROUND

Dynamics of a suspension-slider-air-bearing system are a consideration for harddisk drive (HDD) system research, including suspension and slider designs, servo control system designs etc. The dynamics can be simulated using software. However, there is currently no direct method to characterize the dynamics.

Understanding of the suspension-slider-air-bearing dynamics is typically important for obtaining more stable flying heights, faster slider settling, and more reliable slider-disk interfaces to further improve the performance of hard disk drives. Therefore, the evaluation of the dynamic properties of the system can be a concern. It is typically desired for a slider's fluctuation induced by impacts between the slider and a disk to be rapidly damped out so that successive impacts do not drive the slider motion into resonance and result in a slider crash. Controlling the fluctuation is also desired for a slider with magneto-resistance (MR) elements because the fluctuation can disadvantageously thermally induce MR signal disturbance.

In slider design and suspension design, on one hand, frequency/system responses of the slider and suspension (also known as the Head-Gimbal Assembly HGA) are typically simulated using software programs, such as the so-called ANSYS program, CML Air Bearing Design Program and Air Bearing Surface Solution (ABSolution) Program. On the other hand, experimental measurements can be used instead of software simulations. However, with experimental measurements, there is currently no direct method to obtain a HGA response.

One current experimental method is to provide a bump or scratch on a disk surface and allow the slider to fly over the bump or scratch. The bump or scratch is deliberately made higher than the nominal flying height of the slider. The slider vibration caused by physical contact with the bump/scratch is measured using a laser Doppler vibrometer (LDV) or using a readback signal envelope obtained in relation to the data storage medium of the disk and a vibration response in time domain is first obtained. Thereafter, a system response curve in frequency domain is derived using Fourier transformation based on the vibration response in time domain. Thus, using this indirect method, the free responses of the slider to the bump or scratch can identify the modal frequencies and damping ratios which in turn can lead to identifying dynamic properties of the system. Examples of this method are described in US6275345B1 and Q. H. Zeng et al. in IEEE/ASME Trans. Mechatronics, 3(3), 210-217(1998), "Dynamics of Suspension-Slider-Air-Bearing System-Experimental Study".

Another current experimental method is to fly the slider on the disk at low rotation speeds. Due to partial contact between the slider and the disk, slider vibration is obtained via excitation. Experimental results indicate that as the contact speed increases, the slider vibration modes shift to a higher frequency. Without initiating at least partial contact, some of the system's intrinsic vibrations can not appear, ie. only at least partial contact can excite the intrinsic vibrations. Examples of this method are described in Bo Liu et al. in IEEE Transactions on Magnetics, .vol. 35, no. 5, September 1999, 2463-2465, "An Experimental Study of Slider Vibration In Nanometer Spaced Head-Disk Interface" and US7119979B2.

In the above methods, physical contact with the disk or bump/scratch on disk is used to apply excitation to the slider. Although physical contact causes the slider response change, one problem is that physical contact typically damages both the slider and disk. In the above methods, the sharper the impact (ie. contact on disk or on the bump), the more frequency components are contained and therefore, the more modes are excited, but the more destructive the techniques become.

Another problem is that contact can not excite vibration over all frequencies within a desired band because the impact is not ideal. In other words, the impact does not contain all frequency components (ie. only in a finite frequency band). Due to the non-uniform excitation in a limited band, only some resonance modes, rather than the whoie band response, can be obtained. Therefore, some resonance mode responses can be missed out.

Yet another problem of measurements using physical contact is that the slider vibration frequency increases with the interaction between the slider and the disk surface.

Furthermore, the above methods can only provide a relatively weak reference value of relative amplitudes at different resonant modes. Also, the above methods cannot obtain a single frequency response, ie. a measurement of the response at a selected (single) frequency. In addition, the experimental measurements provided by the above methods are typically relatively low resolution.

Therefore, there exists a need for a method of measuring a frequency response of a slider and a system for measuring a frequency response of a slider that seek to address at least one of the above problems.

SUMMARY

In accordance with an aspect of the present invention, there is provided a method of measuring a frequency response of a slider, the method comprising the steps of flying the slider above a rotating disk; applying a periodic driving force to the slider to excite a vibration of the slider; measuring a vibration amplitude of the slider at a frequency of the periodic driving force; and tuning the periodic driving force over a frequency range to measure said frequency response of the slider. The tuning may further comprise varying a power of periodic driving force.

Applying the periodic driving force may comprise applying heat to a coating on the rotating disk to vary a distance between a top surface of the coating and the slider as a result of thermal expansion of the coating in an area underneath the slider.

The periodic driving force may be applied via an airbearing existing between the slider and the top surface of the coating as a result of the rotation of the disk.

The applying of the heat to the coating may comprise directing a laser beam onto the coating in said area underneath the slider.

The method may further comprise modulating the laser beam to vary the heating of the coating such that a resulting periodic variation in the distance of the slider and the top surface of the coating excites the vibration of the slider.

The laser beam may be modulated to have a sinusoidal, square, sawtooth or triangle waveform.

Applying the periodic driving force may comprise applying an alternating current (AC) voltage between the rotating disk and the slider to vary a distance between a top surface of the disk and the slider as a result of electromagnetic interaction.

The measuring of the vibration amplitude of the slider at the frequency of the periodic driving force may comprise using a laser Doppler vibrometer.

The slider may comprise a magnetic sensor and the top surface of the disk may comprise a magnetic layer containing written data. The method may further comprise obtaining a readout signal from the magnetic sensor; filtering the readout signal using a filter to obtain an envelope of the readout signal; and measuring the vibration amplitude of the slider by applying a Wallace spacing loss (WSL) equation:

where, A _d i is the readout signal amplitude at a first distance of d-i between the sensor and the magnetic layer, A _d2 is the readout signal amplitude at a second distance of d ₂ between the sensor and the magnetic layer, and f is a data signal frequency of the written data.

In accordance with another aspect of the present invention, there is provided a system for measuring a frequency response of a slider, the system comprising means for applying a periodic driving force to the slider to excite a vibration of the slider during flight of the slider above a rotating disk; means for measuring a vibration amplitude of the slider at a frequency of the periodic driving force; and means for tuning the periodic driving force over a frequency range to measure said frequency response of the slider.

The means for tuning the periodic driving force may be further arranged to vary a power of periodic driving force.

The means for applying the periodic driving force may be further arranged to apply heat to a coating on the rotating disk to vary a distance between a top surface of the coating and the slider as a result of thermal expansion of the coating in an area underneath the slider.

The periodic driving force may be applied via an airbearing existing between the slider and the top surface of the coating as a result of the rotation of the disk. For the applying of the heat to the coating, the means for applying the periodic driving force may be further arranged to direct a laser beam onto the coating in said area underneath the slider.

The means for tuning the periodic driving force may be further arranged to modulate the laser beam to vary the heating of the coating such that a resulting periodic variation in the distance of the slider and the top surface of the coating excites the vibration of the slider.

The laser beam may be modulated to have a sinusoidal, square, sawtooth or triangle waveform.

The means for applying the periodic driving force may be further arranged to apply an alternating current (AC) voltage between the rotating disk and the slider to vary a distance between a top surface of the disk and the slider as a result of electromagnetic interaction.

The means for measuring the vibration amplitude of the slider may comprise a laser Doppler vibrometer.

The slider may comprise a magnetic sensor and the top surface of the disk may comprise a magnetic layer containing written data.

The system may further comprise a readout signal circuit for obtaining a readout signal from the magnetic sensor; a filter for filtering the readout signal to obtain an envelope of the readout signal; and the means for measuring the vibration amplitude of the slider may be arranged to apply a Wallace spacing loss (WSL) equation:

ΔlL = eχn[ l-J 2^1

where, A _d i is the readout signal amplitude at a first distance of d-i between the sensor and the magnetic layer, A _d2 is the readout signal amplitude at a second distance of d ₂ between the sensor and the magnetic layer, and f is a data signal frequency of the written data.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1 is a schematic diagram illustrating a setup for measuring a slider's dynamic response in an example embodiment.

Figure 2 is a schematic diagram illustrating a blow-up view of the slider flying over a disk in the example embodiment.

Figure 3 is a schematic diagram of an experimental set-up in one example embodiment.

Figure 4 shows a series of sinusoidal optical waveforms for modulation frequencies of about 1OkHz, 5OkHz, 10OkHz, 15OkHz for use in the example embodiment.

Figure 5 is a graph showing laser Doppler vibrometer (LDV) response amplitudes measured at different sinusoidal modulation frequencies of about 5OkHz, 6OkHz and 7OkHz.

Figure 6 shows different LDV response amplitudes at different laser peak powers at a fixed modulation frequency.

Figure 7 is a graph of measured LDV response amplitude (V) vs laser peak power (mW) in an example embodiment. Figure 8 is a frequency response graph of measured response magnitude (V) vs frequency (kHz) in an example embodiment.

Figure 9 shows the single frequency responses at frequencies of about 112kHz, 113kHz, 114kHz, 115kHz, 116kHz, 117kHz and 118kHz for a 60Gb/in ² slider in an example embodiment.

Figure 10 is a schematic drawing illustrating a setup for measuring a slider dynamic response in one example embodiment.

Figure 11 (a) is a schematic drawing illustrating a setup for measuring a slider dynamic response in one example embodiment.

Figure 11(b) shows a readback signal in the example embodiment.

Figure 11(c) shows an envelope of the readback signal in the example embodiment.

Figure 12 is a schematic flowchart for illustrating a method of measuring a frequency response of a slider in an example embodiment.

Figure 13(a) shows a square wave for use in one example embodiment.

Figure 13(b) shows a saw tooth wave for use in one example embodiment.

Figure 13(c) shows a triangular wave for use in one example embodiment.

Figure 14 is a graph showing a dependence of simulated thermal expansion on local temperature increase of a coated layer in one example embodiment.

DETAILED DESCRIPTION The example embodiments described below can provide a direct and substantially non-contact (non-destructive) method to characterize a HGA response in a broad band with relatively high resolution.

Figure 1 is a schematic diagram illustrating a setup for measuring a slider's dynamic response in an example embodiment. A flyable disk 102 is provided and rotatable on a spindle 104. The disk 102 is a single side flyable disk in the example embodiment. A head gimbal assembly (HGA) comprising a slider 106 is provided. In operation, the slider 106 flies over the disk 102 as the disk 102 is rotating on the spindle 104. A modulated laser 108 is directed and focused on the disk 102 to provide a laser beam 110. A laser Doppler vibrometer (LDV) 112 is provided for measuring the slider 106 vibrations.

Figure 2 is a schematic diagram illustrating a blow-up view of the slider 106 flying over the disk 102 in the example embodiment. In the example embodiment, the flyable disk 102 is a one-side disk comprising a coating or coated layer 202 disposed on a glass substrate 204. The coated layer 202 can comprise more than one coated layers. An example media structure for the disk 102 comprising the coated layer 202 is glass/TaMoCr/Co/CoCr/Ru/CoCrPt/C. The coated layer 202 has a thermal expansion property. Figure 14 is a graph showing a dependence of simulated thermal expansion on local temperature increase of the coated layer 202. Thus, when the laser beam 110 is directed on the coated layer 202, the coated layer 202 absorbs energy and is heated up locally. As a result of thermal expansion of the coated material, a bump 206 can be generated on the disk 102 surface underneath the slider 106. Thus, the laser beam 110 induces a thermal distortion (or bump 206) on the surface of the coated layer 202. The bump 206 disappears after the media or coated layer 202 cools down, ie. when the laser beam 110 is removed or the power of the laser beam 110 is reduced.

In the example embodiment, the laser intensity of the laser beam 110 is modulated with a desired waveform, for example, a sinusoidal wave. Thus, there is provided an intensity modulation/tuning to the laser beam 110 in accordance with the applied waveform. As the modulated laser beam 110 heats the media or the coated layer 202 under a magnetic head part (not shown) of the slider 106, the bump 206 appears periodically (due to the applied waveform) and thus, a periodic driving force directly drives/excites vibration of the slider 106 at the frequency of the applied waveform, ie. the flying height (distance between a top surface of the coating and the slider) of the slider 106 changes. In the example embodiment, the periodic driving force is applied via an airbearing existing between the slider 106 and the top surface of the coating as a result of the rotation of the disk. In the example embodiment, the bump height is less than the nominal fly-height of the slider 106.

Figure 3 is a schematic diagram of an experimental set-up in one example embodiment. The principles of operation are substantially identical to those as illustrated in Figures 1 and 2. In the example embodiment, a slider 302 is provided at an underside of a magnetic disc 304. The disc 304 can be rotated using a motor 306. A laser beam 308 is focused on the disc 304 using objective lens 310. A LDV beam 312 is directed to the slider 302 using prisms 314, 316 and focused on the slider 302 for measuring the response of the slider 302. In the example embodiment, the wavelength of the laser is about 405nm and the numerical aperture (NA) of the objective lens 310 is about 0.6.

Figure 4 shows a series of sinusoidal optical waveforms for the modulation frequencies of about 10kHz (see 402), 50kHz (see 404), 100kHz (see 406), 15OkHz (see 408) for use in the example embodiment. In the example embodiment, by keeping the laser maximum power constant and tuning the laser modulation frequency, the HGA system response curve in frequency domain can be directly obtained.

Figure 5 is a graph showing laser Doppler vibrometer (LDV) (compare 112 of Figure 1) response amplitudes measured at different sinusoidal modulation frequencies of about 5OkHz (see LDV response curve 502), 6OkHz (see LDV response curve 504) and 7OkHz (see LDV response curve 506). Due to the slider having intrinsic vibration modes, for the different frequencies, the slider exhibits different response magnitudes, compare amplitude peak 508 of LDV response curve 502, amplitude peak 510 of LDV response curve 504 and amplitude peak 512 of LDV response curve 506.

Figure 6 shows different LDV response amplitudes at different laser peak powers at a fixed modulation frequency. The modulation frequency is about 112 kHz. The graphs show the different LDV response amplitudes measured at about OmW laser peak power (see 602), about 2.5mW laser peak power (see 604), about 5.OmW laser peak power (see 606), about 7.5mW laser peak power (see 608), about 10.OmW laser peak power (see 610), about 12.5mW laser peak power (see 612), about 15.OmW laser peak power (see 614) and about 17.5mW laser peak power (see 616). It is observed that as the laser peak power increases at the fixed modulation frequency, the LDV response amplitude measured is stronger.

Figure 7 is a graph of measured LDV response amplitude (V) vs laser peak power (mW) in an example embodiment. This graph is based on the results obtained from Figure 6. The dependence of the slider vibration amplitude (ie. the measured LDV response amplitude) on the laser peak power can be observed. Indeed, it can be observed that the amplitude response of the slider (ie. the measured LDV response amplitude) shows a substantially linear relationship with the applied laser peak power, within the range applied in the example embodiment. In the example embodiment, within that range, the coated layer 202 temperature increase has a linear relationship with laser power.

Figure 8 is a frequency response graph of measured response magnitude (V) vs frequency (kHz) in an example embodiment. In the example embodiment, a 60Gb/in ² slider HGA is characterised across a range of frequency from OkHz to about 18OkHz. As can be seen from Figure 8, the frequency domain response of the 60Gb/in ² slider is characterised. The response curve 800 shows that there are three main resonant frequencies at about 50 kHz (see 802), about 113kHz (see 804) and about 117kHz (see 806) within the measurement frequency band. There are some other resonant frequencies too below 135kHz. However, when the frequency is higher than 135kHz, the slider response is observed to be weak (see 808).

Figure 9 shows the single frequency responses at frequencies of about 112kHz, 113kHz, 114kHz, 115kHz, 116kHz, 117kHz and 118kHz for a 60Gb/in ² slider in an example embodiment (see curves 902-914 respectively). The detailed slider response for each single frequency is obtained. In addition to the differences in amplitudes, the differences in the frequency domain are clearly shown for each single frequency. At some frequencies, such as 113kHz (see 904) and 118kHz (914), the slider vibrates with the exciting frequency (e.g. displaying one clear peak at the exciting frequency). At some frequencies, such as 115kHz (see 908) and 116kHz (910), the slider's response includes other strong frequency components. For example, at 115kHz (see 908), the other frequency vibration amplitudes are even higher than that at the exciting frequency (compare peaks 916, 918 at frequencies adjacent to 115kH∑). This means that at this frequency 115kHz, the driving force has strongly excited other frequency vibrations. The inventors have recognised that it may be impossible for current methods in the prior art to observe such detailed information. Further, it is appreciated that such information (as shown in Figure 9) can be important for ultra-low fly-height slider designs.

In another example embodiment, an alternating current (AC) voltage is applied between a slider and a rotating magnetic disk to induce slider vibration having a relatively small amplitude. Thus, a periodic driving force can vary a distance between a top surface of the disk and the slider as a result of electromagnetic interaction. The HGA dynamic responses of the slider can be obtained by tuning the AC frequency and recording the slider vibration amplitudes at each frequency.

Figure 10 is a schematic drawing illustrating a setup for measuring a slider dynamic response in one example embodiment. A disk 1002 is rotated using a spindle 1004. A HGA 1006 is provided comprising a slider 1008 flying over the disk 1002. An alternating current (AC) voltage source 1010 is provided for applying an AC voltage between the slider 1008 and the disk 1002 to drive slider vibrations. A LDV 1012 is provided for measuring slider vibration amplitudes. In the example embodiment, when a sinusoidal wave voltage with a small amplitude is applied using the voltage source 1010 between the slider 1008 and the disk 1002, a slider vibration is caused (because the disk 1002 is relatively heavy and stiff such that it is not easy to vibrate as compared to the slider 1008). The vibration of the slider 1008 can be obtained by using the LDV 1012 monitoring signal. The HGA dynamic responses can be obtained by tuning the sinusoidal wave frequency and recording the slider vibration amplitudes at each frequency. Figure 11 (a) is a schematic drawing illustrating a setup for measuring a slider dynamic response in one example embodiment. The principles are substantially identical to the example embodiment described using Figure 10. However, in this example embodiment, the dynamic response of the slider is measured on the basis that the slider comprises a workable magnetic head/read sensor and the disk comprises a workable magnetic media. A magnetic recording media substrate 1102 comprising a magnetic recording layer 1104 is provided. A HGA 1106 is provided comprising a slider 1108 flying over the magnetic recording layer 1104. The media substrate 1102 is rotated with respect to the slider 1108. An alternating current (AC) voltage source 1110 is provided for applying an AC voltage between the slider 1108 and the magnetic recording layer 1104 to drive slider vibrations. A readout signal circuit 1107 is provided to obtain a readout/readback signal from a magnetic read sensor 1109 of the slider 1108.

. In the example embodiment, the circuit-applied AC voltage (from the AC voltage source 1110). is isolated from the readout signal circuit 1107. Therefore, the applied AC voltage does not affect the normal operation of the magnetic read sensor and the readout signal can be obtained as per normal operation.

Figure 11(b) shows a readback signal 1112 in the example embodiment. Figure 11(c) shows an envelope 1114 of the readback signal 1112 in the example embodiment.

In the example embodiment, firstly, a series of high frequency data, such as

01010101 , at a frequency in a range of e.g. a few tens MHz to a few hundreds MHz, is written onto the magnetic recording layer 1104. In normal operations, ie. when there is no AC voltage applied between the slider 1108 and the disk/substrate 1102, the readout signal obtained by the magnetic read sensor of the slider 1108 is a high frequency signal at the same frequency as the written data frequency, and with a uniform amplitude. To measure the slider dynamic response, an AC voltage with a frequency of less than about 1 MHz is applied between the slider 1108 and the disk/substrate 1102. At this time, the amplitude of the readout high frequency signal obtained by the magnetic read sensor of the slider 1108 is modulated with the applied AC frequency because the readout signal amplitude depends on the distance between the read sensor of the slider 1108 and the recording layer surface of the magnetic recording layer 1104. An example of such a signal is shown as signal 1112 in Figure 11(b).

In the example embodiment, a low-pass filter or a band-pass filter can be used to filter out the high frequency data signal and obtain the envelope of the readout signal. In other words, a filter can be used filter out the written high frequency data signal to isolate the slider vibration effect caused by the applied AC voltage. The readout signal amplitude has a nonlinear relationship with the distance between the read sensor and the recording layer surface. An equation to govern the relationship between the readout signal amplitude and the distance is a Wallace spacing loss (WSL) equation:

where, A _d1 is the readout signal amplitude at a distance of d-i between the read sensor and the recording layer surface, and A _d2 is the readout signal amplitude at a distance of d ₂ between the read sensor and the recording layer surface, f is the data signal frequency (or high frequency data signal frequency). Therefore, in the example embodiment, after obtaining the envelope of the readout signal, a mathematical signal processing is used (ie. implemented by software or hardware) to convert the envelope amplitude into slider vibration amplitude.

It will be appreciated by a person skilled in the art that the method of obtaining the slider vibration signal by read sensor readout signals may not be used in combination with the "heating of a coating layer" method because when a magnetic media is heated, its magnetic property typically exhibit changes. Therefore, its readout signal amplitude typically exhibits another relationship with temperature. This can cause a more complicated change in readout signal amplitude when used to derive the slider vibration/displacement.

Figure 12 is a schematic flowchart 1200 for illustrating a method of measuring a frequency response of a slider in an example embodiment. At step 1202, the slider is flown above a rotating disk. At step 1204, a periodic driving force is applied to the slider to excite a vibration of the slider. At step 1206, a vibration amplitude of the slider is measured at a frequency of the periodic driving force. At step 1208, the periodic driving force is tuned over a frequency range to measure said frequency response of the slider.

In the above described example embodiments, a direct characterisation method can be provided. A system response can be mapped using the above described example embodiments by frequency scanning in a desired frequency band and can allow identifying of a desired frequency response. In the above described example embodiments, e.g. a laser modulated in power with a desired frequency or an AC voltage with a desired frequency is used to measure system response at that frequency of interest. The laser modulation frequency or AC voltage frequency can also be set to scan within a desired frequency band to characterize the system response in this frequency band. Thus, the above described example embodiments can allow investigation of system responses and provide a broad band response curve (including resonant modes and amplitudes). Also, the above described example embodiments can provide relatively accurate amplitude values for different resonant modes.

The above described example embodiments can provide a number of advantages over the prior art. The above described example embodiments can provide relatively high resolution measurements, given that the frequency modulation can be tuned. The above described example embodiments can provide a non-destructive characterization method since no components are damaged in the measurements. Further, the above described example embodiments are relatively easy to implement. The above described example embodiments can allow characterising of wide band responses of a HGA (including frequency and amplitude) and any desired frequency response of the HGA can be measured.

Thus, the above described example embodiments can provide a tool for providing useful information e.g. for head-disk interfaces and whole HGA system designs. It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

For example, the laser power for the laser beam can be modulated with a variety of waveforms, such as sinusoidal waves, square waves, triangle waves, sawtooth waves etc. It will be appreciated that depending on the waveform used, initial calibrations may be carried out.

Figure 13(a) shows a square wave suitable for use in one example embodiment. ^• • Figure 13(b) shows a saw tooth wave suitable for use in one example embodiment. Figure 13(c) shows a triangular wave suitable for use in one example embodiment.